New technology is being used to help monitor the extent of dryland salinity threatening large areas of Australia's agricultural zone.
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You will get more from this topic if you have mastered the basics of electromagnetic radiation these links will take you to an annotated list of sites with helpful background information.
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The term 'dryland salinity' strikes fear into the hearts of many
Australian farmers. Some call it the white death because it conjures
up images of lifeless, shining deserts studded with dead trees.
Fears of the 'white death' seem justified. Dryland salinity currently affects more than 5 million hectares of land, mostly in southern Australia and causes damage totalling $270 million each year.
What is dryland salinity?
There are two kinds of soil salinity: dryland salinity (occurring
on land not subject to irrigation) and irrigated land salinity.
Both describe areas where soils contain high levels of salt. Usually, plants and soil organisms are killed or their productivity is
severely limited on affected lands.
Much of Australia's landscape is naturally saline think of the
great salt lakes in our interior. Many of our agricultural lands
also contain vast reservoirs of salt, but normally these are held
deep within the soil profile where they don't affect plant growth. The problem occurs when this salt is brought to the soil surface by rising water tables (Box 1: Salinisation causes and prevention).
Where does the salt come from?
The salt that sits deep in the soil profile may have several sources. In Western Australia, the main source is believed to be the ocean salt is carried inland by the prevailing winds and deposited on the land in rainfall and dust. Over a time scale of millions of years, this process has deposited large amounts of salt in what is now the West Australian wheatbelt.
Some salt in the soil profile may date back even further, to when
the parent rocks themselves were formed. These rocks release salts as they weather. Other possible sources of salt are ancient drainage basins or inland seas that evaporated during arid periods, leaving behind salt deposits that still remain today.
Monitoring the problem
In the past, farmers estimated the extent of salinisation on their
properties in response to questionnaires issued by the Australian
Bureau of Statistics. This method is thought to have underestimated
the extent of salinisation, partly because the definition and
recognition of salinisation varies between farmers. Nor do such
methods provide maps of where the salinity is, where it is spreading
to, or the rate at which it is spreading.
In recent years, scientists have developed new techniques for
monitoring salinity. Most involve what is known as remote sensing. This is the collection of data using devices fitted to an aeroplane, satellite or some other craft located above the Earth's surface. Such technology can be used to gather a range of information related to salinity. Often remote sensing involves cameras that can record electromagnetic radiation particularly visible light and infrared light reflected from the Earth's surface.
Electromagnetic reflections sensing the differences
When the sun's rays made up of electromagnetic radiation of
many different wavelengths strike plants, water bodies, soils and
other features on the Earth's surface, some wavelengths are
absorbed by molecules in these features and some are reflected.
Different features on the Earth's surface will absorb and
reflect different parts of the electromagnetic spectrum depending
on their chemical make-up. In this way, different parts of the
electromagnetic spectrum provide information about the Earth's
surface that may be useful for the detection of salinisation.
Monitoring by aeroplane
Australian scientists have tested a number of techniques to collect
and analyse electromagnetic information. For example, colour infrared film can be used to take photographs from aeroplanes. Different
colours (corresponding to different wavelengths within the infrared
band) will show vegetation under varying levels of stress, which
can then be related to the degree of salinity. Dark-green vegetation
produces a bright red image, light-green foliage a pink image,
barren saline soil a white image, salt-stressed vegetation a reddish-brown image. If such photographs are taken of the same area over different years, changes in the pattern of salinisation can be monitored.
Similarly, video cameras can be used from aeroplanes to collect information in the visible band of the spectrum. The videos show salinity patterns and the way these change over
time.
Another airborne electromagnetic technique makes use of the fact
that electrical conductivity increases with increasing salinity. It involves an aeroplane flying low over the ground. Mounted on board is an electromagnetic transmitter and trailing behind on a cable is a receiver. The transmitter sends out pulses of electromagnetic radiation. When these hit the ground, they induce electrical currents to flow in conductive areas. The decay of these currents produces a magnetic field which is recorded by the receiver trailing behind the aircraft. The recording is then analysed to determine the conductivity of the ground.
Monitoring by satellite
Increasingly, scientists are also using satellite images to analyse
salinity patterns across large areas. Most images are supplied
by a series of scientific satellites known as Landsat. These orbit the Earth, recording information about the electromagnetic radiation reflected by the Earth's surface.
In Landsat satellites, an instrument called a Thematic Mapper
makes regular observations in bands ranging from the visible to
the thermal on each area of the Earth's surface, sending the information back to Earth. Many scientists consider that data produced in this way can be used effectively for the detection and monitoring of salinity, and experimental results support this view (Box 2: Mapping salinity).
Predicting and preventing the advance of salinity
The knowledge gained from the new monitoring techniques, along
with that generated by decades of painstaking field research,
is offering many insights to the causes of salinisation.
Importantly, this is aiding scientists in the development of methods
to predict sites most at risk of salinisation so that preventative
measures such as tree-planting can be taken. Armed with the information such methods will provide, a coordinated community response could succeed in combatting the white death, before it eats out our agricultural heart.
Boxes
1. Salinisation causes and prevention
2. Mapping salinity
Related Nova topics:
Salinity the awakening monster from the deep
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